Guorong Wu,Qiangwen Fan,Wenjie Sun,Zhiwu Yu,Zhiqian Jia,Jianguo Ma,*

1 Jiangxi Province Key Laboratory of Polymer Micro/Nano Manufacturing and Devices,East China University of Technology,Nanchang 330013,China

2 Lab for Membrane Science and Technology,College of Chemistry,Beijing Normal University,Beijing 100875,China

3 State Key Laboratory of Nuclear Resources and Environment,School of Biology,Chemistry and Material Science,East China University of Technology,Nanchang,330013,China

4 Jiangxi Province Key Laboratory of Synthetic Chemistry,School of Chemistry,Biology and Material Science,East China University of Technology,Nanchang 330013,China

5 Foshan (Southern China) Institute for New Materials,Foshan 528247,China

Keywords:Pervaporation Biofuel Polydimethylsiloxane Membranes Metal-organic frameworks

ABSTRACT Pervaporation (PV) is an emerging separation technique for liquid mixture.Mixed matrix membranes(MMMs) often demonstrate trade-off relationship between separation factor and flux.In this study,by changing the organic linkers (2-methyl imidazolate,imidazole-2-carboxaldehyde,2-ethyl imidazolate),ZIF-8,ZIF-90 and MAF-6 were prepared and filled in polydimethylsiloxane (PDMS) membranes for dealcoholization of 5% (mass) n-butanol solution,and the membranes properties and pervaporation performances were adjusted.Compared with the pure PDMS membrane,the addition of ZIF-8 resulted in a 9% increase in flux (1136 g·m-2·h-1) and a 22.5% increase in separation factor (28.3),displaying antitrade-off effect.For the MAF-6/PDMS MMMs (2.0% mass loading),the pervaporation separation index(PSI) and separation factor were 32347 g·m-2·h-1 and 58.6 respectively (increased by 34% and 154% in contrast with that of the pure PDMS membrane),and the corresponding permeation flux was 552 g·m-2·h-1,presenting great potential in the removal butanol from water.It was deduced that the large aperture size combined with moderate hydrophobicity of metal-organic frameworks (MOFs)favor the concurrent increase in permeability and selectivity.

1.Introduction

Pervaporation(PV)is a prospective membrane-based separation technique.Compared with conventional separation processes,PV has many outstanding merits,such as energy effective,ecofriendly and easy-to-operate [1].Polydimethylsiloxane (PDMS) is an elastomeric material,exhibiting superior thermal stability,high hydrophobicity and excellent film-forming ability [2,3].PDMS has been widely used in PV for the recovery of bio-butanol[4]and bioethanol [5] from aqueous streams.To mitigate the plasticization effects,mixed matrix membranes (MMMs),such as silicate/PDMS[6],zeolite/PDMS[7],were applied to heighten the stability,selectivity and fluxes of PDMS membranes.Nevertheless,the compatibility between PDMS and inorganic fillers is often not pleased,leading to the trade-off relationship or unsatisfactory PV performance.

Metal-organic frameworks(MOFs),as an emerging class of porous inorganic-organic hybrid materials,have the advantages of permanent porosity,chemical versatility,high surface area and designable framework topologies[8-11].Among MOFs,zeolitic imidazolate frameworks(ZIFs),also called zeolitic metal azolate frameworks(MAFs),composed of deprotonated polyazaheterocycles(e.g.pyrazoles,imidazoles,and triazoles) and metal cations,have received considerable attention due to their zeolite-like permanent porosity,high thermal,chemical stability and uniform pore size[8].

ZIF-8,ZIF-90 and MAF-6 are composed of Zn2+and 2-methyl imidazolate,imidazole-2-carboxaldehyde and 2-ethyl imidazolate(Fig.1),with the pores aperture size of 0.34,0.35 and 0.76 nm respectively [12-14].In the present study,ZIF-8,ZIF-90 and MAF-6 were fabricated and incorporated in PDMS matrix for the removal ofn-butanol from water.The influences of the organic linkers on hydrophilicity,sorption degree and pervaporation performance of Zn-MOFs/PDMS MMMs were studied.

2.Experimental

2.1.Materials

Fig.1.Organic linkers of Zn-MOFs.

Hydroxyl polydimethylsiloxane (PDMS,viscosity of 50,000),dibutyltin dilaurate (98.0%) and tetraethyl orthosilicate (TEOS,98%)were provided by Jinan Xingfeilong chemical factory.Concentrated ammonia (25%-28%),methanol (99.5%),ethanol (99.7%),nbutanol (99.0%),N,N-dimethylformamide (DMF,99.5%),pyridine(99.0%)andn-heptane(99.0%)were purchased from Beijing Chemical Factory.2-methylimidazole (99.0%,2-MIM) and 2-ethylimidazole (99.0%,2-EIM) were bought from Acros.Zn(OH)2(99.0%)was provided from Tianjin Guangfu fine chemical research institute.Zn(NO3)2·6H2O (99.0%) was acquired from Xilong Chemical Co.,Ltd.Imidazole-2-carboxaldehyde (ICA,98.0%) was bought from Admas Reagent Co.,Ltd.All the above chemicals were analytical grade.

Polypropylene (PP) microfiltration membranes (pore size of 0.10 μm,diameter of 50 mm) were bought from Taoyuan Medical Chemical Instrument Factory.Polyvinylidene fluoride(PVDF)ultrafiltration membranes(thickness of 77 μm,molecular weight cut off of 100,000-150,000) were purchased from Beijing Saipuruite.

2.2.Synthesis of Zn-MOFs

Preparation of ZIF-8:40 ml of 2-MIM(0.3844 g)methanol solution was added into 40 ml of Zn(OH)2(0.1980 g) concentrated ammonia solution.The resultant slurry was stirred at room temperature (RT) for 5 h,filtered,washed with methanol,and then dried at 120 °C [13].

Preparation of ZIF-90:ICA (10.1370 g) was dissolved in DMF(633 ml) at 60 °C,and the solution was filtered.After cooled to RT,6.1 ml of pyridine was added,and then 633 ml of Zn(NO3)2-·6H2O (3.9270 g) DMF solution was introduced and stirred for 20 h.Then methanol (900 ml) was mixed and stirred for 20 h.The as-obtained precipitate was centrifugated (12,000 r·min-1),washed with DMF 3 times and dried at 60 °C for 12 h in vacuum oven [14,15].

Preparation of MAF-6:40 ml of Zn(OH)2(0.1980 g) concentrated ammonia solution was quickly added into 6 ml of 2-EIM(0.3840 g) methanol solution (containing 1.3 ml of cyclohexane),stirred for 0.5 h,filtered,washed with methanol,and dried at 120 °C [13].

2.3.Preparation of Zn-MOFs/PDMS MMMs

Zn-MOFs were mixed with 10 ml ofn-heptane,then stirred and ultrasonicated.Afterwards,the 10 ml 12.5% (mass) PDMSnheptane solution was introduced and stirred for 3 h.Dibutyltin dilaurate(0.145 ml)as a catalyst and TEOS(0.27 ml)as a crosslinking agent were added and stirred for 3 min.Then the solution was cast on PVDF membranes with cast knife (space of 370 μm),dried at RT for 12 h,and then 80 °C under vacuum for 4 h [16].

2.4.Pervaporation

The pervaporation apparatus was presented in our previous paper [1].In this study,5% (mass)n-butanol solution was applied in the PV process,the temperature of feed is 30°C,and the pressure in the permeate side was 14 Pa.In addition,the permeates were analyzed by GC-14C(Thermal conductivity detector,GDX-103 column).The permeation flux(J),separation factor(β),pervaporation separation index (PSI) are calculated as follows [1,17],

whereQ,Aand Δtare the permeates mass(g),effective membrane area (m2) and time (h),respectively;XandYrepresent the weight fraction in the permeate and feed sides of the membrane,and the subscriptsiandjdenoten-butanol and water.To elaborate the intrinsic PV properties,the permeance (GPU) and selectivity (αij)are calculated [2,18-20] as,

whereJiis the componentimolar flux(mol·cm-2·s-1),Pi0andPi1are the corresponding partial pressure on the feed and permeate sides of the membrane,xi,γiandare its molar concentration,activity coefficient,and saturated vapor pressure,respectively.Pi1was calculated by the total pressure and gas composition in the permeate side.γ was obtained with the aid of Aspen Plus 7.2 software,andwas calculated using the Antonine equation.

2.5.Characterization

Scanning electron microscope (SEM) images of Zn-MOFs and membranes were obtained by an S-4800 field-emission scanning electron microscope.Powder X-ray diffraction (PXRD) patterns were collected using a Shimadzu XRD-6000 diffractometer.Fourier transform infrared spectrometry (FTIR) spectra of Zn-MOFs and membranes were acquired using a Nicolette 380 FTIR spectroscopy.The contact angles of membranes were carried out by OCA 20 optical contact angle meter at room temperature.N2adsorption isotherms of Zn-MOFs were carried out by Nitrogen sorption (Boynton Beach,Florida).In the characterization of sorption degree [1],the dry membranes were weighed (Wd),and then soaked inn-butanol at RT for 24 h.The sorption degree(SD)is calculated as follows:

In the measurement of partition coefficient ofn-butanol[16]in pure PDMS and MMMs,the membranes was immersed in 5.0%(mass)n-butanol solution at RT for 24 h,and the partition coefficient ofn-butanol in pure PDMS membrane is defined as [21],

wheremsolandmPDMSis then-butanol concentration in solution and PDMS respectively.Similarly,the partition coefficient ofnbutanol in MMMs is defined as,

The MMMs contain the PDMS matrix phase and the Zn-MOFs phase,and then-butanol concentration in MMMs (kg·kg-1) can be expressed as,

wheremZn-MOFsis then-butanol concentration in Zn-MOFs,K′′is the partition coefficient ofn-butanol in Zn-MOFs,MPDMSandMZn-MOFsare PDMS and Zn-MOFs mass fraction in MMMs,respectively.The contact angle,sorption degree and PV performance data were the average value of triplicate determinations,and the relative errors were calculated.

3.Results and Discussion

3.1.Characterization of Zn-MOFs

Fig.2 shows that ZIF-8,ZIF-90 and MAF-6 are about 30 nm,1.411 μm and 500 nm in average size according to the scale bar at the bottom right.In the FTIR spectra (Fig.2(a)),the peaks at about 2974 cm-1and 3375 cm-1represent=C-H and -N-H stretching vibration in imidazole ring.For ZIF-90,the peak at 1672 cm-1corresponds to the stretching vibration of aldehyde.For MAF-6,1248 cm-1is assigned to the ethyl stretching vibration[22].PXRD patterns show that the particles display good crystallinity(Fig.2(b)),consistent with the literature[13,15].As shown in Fig.3,the BET surface areas of ZIF-8,ZIF-90 and MAF-6 are 1248 m2·g-1,662 m2·g-1and 1602 m2·g-1respectively (Fig.4).

3.2.Characterization of MMMs

Fig.5 presents the SEM images of the pure PDMS(Fig.5(c)),ZIF-8/PDMS (Fig.5(d)),ZIF-90/PDMS [Fig.5(e)] and MAF-6/PDMS membranes (Fig.5(b),(f)) with 4% (mass) loading of Zn-MOFs.As shown,both the pure PDMS (Fig.5(a)) and Zn-MOFs/PDMS(Fig.5(b)) membranes surfaces are smooth,and no apparent defects are seen in MMMs,and Zn-MOFs are dispersed in the PDMS matrix.From Fig.5(c)-(f),the thickness of Zn-MOFs/PDMS layer(40 μm) is slightly higher than that of pristine PDMS (26 μm)because of the incorporation of Zn-MOFs.

The FTIR spectra of membranes are depicted in Fig.6.For pure PDMS membrane,the bands at 790 cm-1and 1076 cm-1,1009 cm-1and 1258 cm-1,and 2851 cm-1and 2962 cm-1correspond to the symmetric and asymmetric stretching vibration of C-C,Si-O-Si and -CH3respectively.The peak at 864 cm-1is from Si-C stretching vibration [24].For MMMs (10% mass loading),the stretching vibration of C-H (2906 cm-1) and C-N(1320 cm-1) in imidazole ring appear.For ZIF-90 MMMs,the C=O stretching vibration (1675 cm-1) can be observed [25,26].

Fig.2.SEM images of Zn-MOFs.(a) ZIF-8.(b) ZIF-90.(c) MAF-6.

Fig.3.The insert structure models were cited from literature [13,23].

Fig.4.N2 adsorption/desorption isotherms of Zn-MOFs.

Fig.5.SEM analyses of membranes.(a) Surface of pristine PDMS.(b) Surface of MAF-6/PDMS MMMs.(c)Cross-section of pristine PDMS.(d)Cross-section of ZIF-8/PDMS MMMs.(e)Cross-section of ZIF-90/PDM MMMs.(f)Cross-section of MAF-6/PDMS MMMs.The Zn-MOFs mass loading in MMMs was 4%.

Fig.6.FTIR spectra of membranes.

Looking at Fig.7(a),the water contact angle (CA)of pure PDMS membrane is 109°.The CA of MMMs is generally higher than that of pure PDMS.With the increasing mass loading (0.5%,1.0%,2.0%,3.0% and 4.0%) of ZIF-8 and MAF-6,the CA of MMMs gradually rises,while that of ZIF-90/PDMS MMMs declines.Compared with the Table 1,the CA of MMMs,with the same loading Zn-MOFs,is in the order of MAF-6/PDMS ＞ZIF-8/PDMS ＞ZIF-90/PDMS,which is in good agreement with the sequence of hydrophobic constants of ligands.The ethylimidazole is more hydrophobic than methylimidazole,and ZIF-8 possesses a hydrophobic pores surface but a hydrophilic crystal surface,while MAF-6 exhibits high hydrophobicity on both internal pores and external crystal surfaces[13].

Fig.7.Effects of Zn-MOFs loading on the membranes.

Table 1Hydrophobic constant of substituent of ligands

Sorption degree may present a quantitative characterization of membrane affinity for a solution.As can be seen from Fig.7(b),with the increasing mass loading (0.5%,1.0%,2.0%,3.0% and 4.0%),the sorption degrees of ZIF-8 and MAF-6 MMMs inn-butanol gradually rise,while that of ZIF-90 MMMs decline.Also,the sorption degree of MAF-6/PDMS is much higher than those of the ZIF-8 and ZIF-90 MMMs with the same loading as well as that of the pristine PDMS,which is related to the high pores volume and strong hydrophobicity of MAF-6 (Table 2).

Table 2Structure parameters of Zn-MOFs [12,13]

3.3.Pervaporation performance

As shown in Fig.8,for the pure PDMS membrane,the butanol/water separation factor is 23.1,and the flux is 1044 g·m-2·h-1.For ZIF-8/PDMS MMMs,with increasing ZIF-8 loading,the separationfactor and permeation flux firstly enhance and then fall,and show a maximum at 0.5% mass loading (28.3,1136 g·m-2·h-1),demonstrating the anti-trade-off effect,which is consistent with literature report [18].For ZIF-90/PDMS and MAF-6/PDMS MMMs,with the increasing loading of Zn-MOFs,the permeation fluxes firstly decrease and then rise.For 2.0% (mass) loading of ZIF-90 and MAF-6,the maximum separation factors are 22.7 and 58.6 (1.09 and 2.54 times of pure PDMS membrane)respectively,and the corresponding minimum permeation fluxes are 932 and 552 g·m-2·h-1(0.9 and 0.5 times of pure PDMS membrane) respectively.

Fig.9.Effects of Zn-MOFs loading on permeance and selectivity in PV.

Fig.10.Effects of Zn-MOFs loading on PSI.

Fig.9 shows the intrinsic pervaporation performance of the above membranes.It can be seen that,the selectivity,water permeance and butanol permeance of pure PDMS membrane are 7.6,5176 GPU and 39,272 GPU.For MMMs,with the addition of MOFs,the selectivity andn-butanol permeance of ZIF-8/PDMS MMMs display maximum at 0.5%(mass)loading and the water permeance almost remains unchanged.Besides,with the increased loading of MAF-6,the butanol and water permeances only slightly decline,and the selectivity shows maximum (19.6) at 2% (mass) loading,about 2.57 times that of pure membrane.Too high loading (＞2%(mass)) may result in particles agglomeration and then reduced selectivity.

From Fig.10,the PSI of pure PDMS membrane is 24148 g·m-2·h-1.As the loading of ZIF-8,ZIF-90,and MAF-6 increases,the PSI of MMMs firstly boosts and then declines owing to the Zn-MOFs agglomeration,and displays maximum of 32,149,21,134 and 32,347 g·m-2·h-1(1.33,0.88,and 1.34 times that of the pristine PDMS membrane)respectively at 0.5%,2.0%and 4.0%(mass)loading.For Zn-MOFSloading between 1%and 4%,the PSI of MMMs,with the same loading of Zn-MOFs,is in the order of MAF-6/PDMS ＞ZIF-8/P DMS ＞ZIF-90/PDMS,which is agreement with water contact angel and sorption degree.Thus,MAF-6/PDMS MMMs exhibit larger separation factor among the three MMMs.

The kinetic size ofn-butanol(0.505 nm)is larger than the aperture size of ZIF-90 (0.35 nm),leading to the restriction of sorption and diffusion ofn-butanol through the channels of MOFs and then the decreased selectivity,flux and butanol permeance (Figs.8,9).For MAF-6,the large aperture size (0.76 nm) [16] and strong hydrophobicity are beneficial to the sorption ofn-butanol,resulting in increased separation factor (Figs.7,8).However,the too strong hydrophobicity,especially in the internal pores of MAF-6,may inhibit the diffusion of the two polar components,leading to low fluxes.It can be deduced that,the moderate hydrophobicity of MOFs may favor the concurrent increase in flux and separation factor,breaking the trade-off relationship.Thus,this work provides a screening criterion for MOFs in MMMs pervaporation.The search of MOFs with large aperture size and suitable hydrophobicity for dealcoholization is now conducting in our group.

Table 3 compares the MMMs for removaln-butanol from water reported in the literature.As shown,ZIF-8/PDMS MMMs demonstrate anti-trade-off effect owing to the addition of ZIF-8,and MAF-6/PDMS MMMs exhibit high separation factor because the incorporated MAF-6 particles not only enhance the sorption ofnbutanol,but also do not hinder the diffusion through membranes.

Table 3Comparison of PDMS MMMs for removal n-butanol from water

4.Conclusions

The properties and PV performance of Zn-MOFs/PDMS MMMs were successfully tuned by altering the organic ligands (2-methyl imidazolate,imidazole-2-carboxaldehyde,2-ethyl imidazolate).As the hydrophobic constants of organic ligands of Zn-MOFs increase,the hydrophilicity of MMMs drops while the sorption degree of MMMs inn-butanol rises.Compared with the pure PDMS membrane,the addition of ZIF-8 resulted in a 22.5% increase in separation factor(28.3)and a 9%increase in flux(1136 g·m-2·h-1),displaying anti-trade-off effect.Additionally,MAF-6/PDMS MMMs present excellent separation performance for the butanol-water system.For the MAF-6 loading of 2.0% (mass),the MMMs shows maximum separation factor (58.6) and PSI (32347 g·m-2·h-1) (increased by 154%and 34%respectively in contrast to the pure PDMS membrane),and the corresponding flux is 552 g·m-2·h-1,providing great potential in recovering dilute butanol from water solution.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The works was supported by the National Natural Science Foundation of China (Nos.22008028,22102022 and 22166002),the Opening Project of Jiangxi Province Key Laboratory of Polymer Micro/Nano Manufacturing and Devices (PMND202003) and Foshan (Southern China) Institute for New Materials(2021AYF25015),State Key Laboratory of Nuclear Resources and Environment of East China University of Technology (NRE2021-16),and the Training Program of National College Students Innovation and Entrepreneurship (202110405009).